1. Field of the Invention
This invention relates to a ball mill of the vertical conical shape for pulverizing cement raw material, clinker, etc. 2. Description of the Prior Art
FIG. 1 shows a vertical conical ball mill of the prior art comprising a rotary container 1 of a substantially inverted frustoconical shape, and a fixed cover 2 arranged in a manner to enclose an upper portion of the rotary container 1. The rotary container 1 contains therein a multiplicity of grinding media 3 which have centrifugal forces exerted thereon as the rotary container 1 rotates, and which ascend along a side wall 1a of the rotary container 1 until they reach the fixed cover 2, where the grinding media 3 are moved inwardly by forces of inertia along guide vanes 4 on an inner surface of the fixed cover 2, and descend to pulverize material 6 in the rotary container 1 fed through a material feeding chute 5 located in an upper portion of the fixed cover 2.
The pulverizing mill apparatus of the aforesaid construction is operative to discharge the pulverized material 6 from the apparatus by means of air currents through a duct 8 connected to a central portion of the fixed cover 2. To allow the rotary container 1 to rotate, it is necessary to provide a gap 9 between the rotary container 1 and the fixed cover 2. However, to keep the pulverized material 6 from being discharged from the apparatus through the gap 9 and scattered to the outside, it is essential that the gap 9 be minimized.
Centrifugal forces of high magnitude are exerted on the pulverized material 6 in the vicinity of the gap 9 to urge the material 6 to fly out of the apparatus. Thus the conical pulverizing mill apparatus of the prior art suffers the disadvantage that continuous operation cannot be performed in a stable manner due to deposition of the particles of pulverized material 6 on the parts of the container 1 and cover 2 facing the gap 9 and blocking of the gap 9 by broken pieces of the steel balls which are grinding media 3 or other foreign matter.
To discharge the particles of pulverized material 6 by means of the air currents 7 in a direction oriented upwardly from the apparatus requires the use of a blower of a large capacity and high pressure capable of developing a large volume of air currents. This is not desirable from the point of view of energy saving.
This type of pulverizing mill apparatus is suitable for use in obtaining pulverization to material of a particle size of below several millimeters in such a manner that the residual portion of the material is about 1% residue on a sieve of 88.mu. and about 10% residue on a sieve of 44.mu.. However, the apparatus is not suitable for coarsely crushing of coarse material of several tens of millimeters or of up to one hundred and several tens of millimeters in particle size to a particle size of 3-4 mm.
A tube mill having two to three chambers is available as a ball mill performing coarse crushing and pulverizing in a single device. This device only relies on impact crushing by the circulatory behavior of steel balls which are crushing media, so that it is very low in pulverizing efficiency.